1. Field of the Invention
The present invention relates to a system and method for reducing the cycle time of an injection molding machine. More particularly, the invention relates to an improved device and method for detecting the presence of an article on a mold of an injection molding machine, and controlling the operation of the device on the basis of this detection. The invention further relates to a device and method for removing molded articles from a mold of an injection molding machine before the articles are fully cooled. Both of these devices and methods, when used individually or together in an injection molding machine, reduce the machine's injection cycle time.
2. Description of Related Art
Containers are commonly made by blow molding a parison or preform that is made from polyethylene terephthalate (PET) material. The PET preheat and blow parisons are commonly manufactured by injection molding equipment. Containers may be injection molded in high-volume, multi-cavity molds.
It is important to reduce the overall cycle time for several reasons: (1) greater efficiency and cost-competitiveness; (2) reduced resin degradation due to prolonged residence time in the mold (specifically, acetaldehyde in PET parts); and (3) improved visual part quality (crystallization, seen as cloudy regions in the molded parts, may occur if cycle times are excessive).
Problems may also occur if articles or portions of articles are left on the mold core pins after the ejection cycle. Portions of articles may remain on the core pins due to (1) failure of the ejection system, (2) breakage of the parison during stripping from the cores, or (3) a "short shot" due to insufficient plasticized material being supplied to the mold cavities. If any molded portions remain on the core pins when the injection mold halves close, the injection mold may be damaged or the next successive part may be defective. To prevent this from happening, a conventional injection molding machine is set-up such that the device (e.g., a take-out plate) used to remove the molded parts from the core pins does not leave the molding area until it receives a signal from the injection molding machine controller that all the molded articles (or portions) have been removed from the core. This conservative approach lengthens the total molding injection cycle time. It also does not confirm that all of the molded articles are on the take-out plate and off the mold core pins. Thus, it would be desirable to reduce the injection cycle time by removing this conservatism from the injection molding process.
To further reduce cycle time, it would be desirable to remove the molded articles from the mold core before they are fully cooled. However, in this state, the parts are soft and malleable and susceptible to surface damage and mechanical deformation. Thus, there is also a need to develop a way to extract molded parts while they are still warm to reduce total cycle time.